Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is an important enzyme in the photosynthetic process. This enzyme incorporates CO2 into plants during photosynthesis. Atmospheric oxygen competes with CO2 as a substrate for Rubisco, giving rise to photorespiration and making Rubisco the rate-limiting step in photosynthesis.
Rubisco Activase (RCA) is a protein that catalyzes the activation of Rubisco, which in turn, regulates photosynthesis by initiating photosynthetic carbon reduction and photorespiratory carbon oxidation. The Rubisco Activase enzyme catalyzes the release of ribulose-1,5-bisphosphate (RuBP) from Rubisco. This newly unoccupied site on Rubisco is now free to bind the CO2 and Mg2+ activators in order for photosynthesis to proceed. Rubisco Activase is also responsible for releasing sugar phosphate inhibitors from Rubisco and restores Rubisco catalytic activity. Thus, if Rubisco Activase is impaired, Rubisco remains inactive and photosynthesis slows.
Rubisco Activase is thermo-labile and thus has decreasing activity with increasing temperatures. As a result, the photosynthetic process slows due to the lack of Rubisco activation. The Rubisco Activase enzyme denatures under increased temperatures, thus rendering the enzyme unable to convert inactive Rubisco to the active form. Arabidopsis contains two RCA isoforms, the short thermolabile (RCA1) and the long relatively thermostable (RCA2) forms that are generated by alternative splicing of pre-mRNA (Werneke, et al., (1989) Plant Cell 1:815-825).
Crop plants grown in hot climates could benefit from increasing photosynthetic levels. Accordingly, if the rate limiting step in photosynthesis could be made more heat tolerant, crop plants could more easily grow in these climates.